With global warming, governments around the world are becoming more and more demanding on energy conservation and emission reduction. Therefore, finding new energy sources to replace fossil fuels has become an urgent issue to resolve.
Solar energy is a clean, pollution-free and inexhaustible energy source. At present, solar energy is utilized primarily via its conversion into electricity by solar modules. The electricity is then used to power electric water heaters, electric vehicles, and satellite components.
A solar module refers to a photovoltaic device generating electricity directly from light, particularly, from sunlight. At present, common examples of solar modules include modules that accommodate crystalline silicon solar cells such as monocrystalline silicon solarcells, polycrystalline silicon solar cells, and nano-silicon solar cells, and thin-film solar cells such as non-crystalline thin-film silicon solar cells, cadmium telluride thin-film solar cells, and copper indium gallium selenide thin-film solar cells.
A crystalline silicon solar module mainly comprises a back sheet, an encapsulant, a photovoltaic circuit and a front sheet. The structure of a thin-film solar cell mainly comprises a photovoltaic circuit placed on a substrate (for example, a glass sheet, polymer sheet or stainless steel sheet), an encapsulant placed on the other surface of the photovoltaic circuit and the substrate, and a front sheet or back sheet placed on the encapsulant.
The encapsulant is used in solar modules to bond the front sheet, the photovoltaic circuit and/or the back sheet together. In a lamination operation carried out at about 150° C., a molten encapsulant flows into voids in a solar cell to encapsulate the solar cell.
At present, commonly used encapsulation materials include ethylene-vinyl acetate copolymers and ionic polymers. However, ionic polymers have disadvantages, including high prices, difficulties in processing and very high hardness. In contrast, ethylene-vinyl acetate copolymers (EVA) have advantages of ease of processing, moderate prices and appropriate flexibility. Therefore, they are the most commonly used polymeric encapsulation materials at present.
The primary role of the front sheet in a solar module is to protect solar cells against mechanical impact and weathering. In order to make full use of the incident light, the front sheet must have a high light transmittance in a certain range of the spectrum (e.g., from 400 to 1,100 nm for crystalline silicon solar cells). The front sheet of prior art solar modules is mainly formed of glass (usually low-iron, tempered flint glass with a thickness of 3 to 4 mm) or polymeric materials.
The primary role of the back sheet in a solar module is to protect the solar cells and the encapsulant against moisture and oxidation. During the process of assembling a solar module, the back sheet is also used as a mechanical shield to prevent scratches and serves as an insulator. Therefore, the back sheet is required to have excellent moisture barrier properties and flexibility, and a good bonding strength with the encapsulant to prevent oxygen and moisture from penetrating into the solar module.
A common back sheet of a solar module is a multi-layer laminated film, mainly comprising a substrate (e.g., a polyester substrate) and a bonding layer on one surface of the substrate.
Although the above-described laminated film has excellent moisture and oxygen barrier properties, the bonding strength between the substrate (e.g., a polyester substrate) and the bonding layer is not completely satisfactory. Sometimes, delamination may take place at the interface between the two layers.
Moreover, the bonding strength between the bonding layer and the encapsulant such as ethylene-vinyl acetate copolymer is also not completely satisfactory. There is still room for improvement.
Therefore, there is a need to provide a laminated film, which has excellent moisture and oxygen barrier properties, excellent bonding strength between its layers, excellent bonding strength with the layer of polyethylene-vinyl acetate resin that serves as an encapsulant for solar cells, and minimizes the use of adhesives and organic solvents.